ABSTRACT In the previous project period of this grant, we investigated the transcriptional mechanisms of CD8+ T cell ?exhaustion?, a hyporesponsive state observed under conditions of sustained antigen stimulation in cancer and chronic viral infections. Exhausted T cells show decreased proliferation and cytokine production, and upregu- late inhibitory cell surface receptors including CTLA4, PD-1, LAG3 and TIM3. Antibodies to these receptors reverse T cell exhaustion, and their administration to cancer patients forms the basis for ?immune checkpoint blockade?, a strategy that has recently been remarkably successful in cancer immunotherapy. Combinations of antibodies to inhibitory receptors show greater efficacy than administration of individual antibodies alone, consistent with the fact that exhausted T cells typically express several inhibitory receptors. Thus understanding the molecular mechanisms that lead to inhibitory receptor expression and T cell ?exhaustion/ dysfunction? would complement and enhance the effects of therapies that target combinations of individual inhibitory receptors. We previously showed that the transcriptional programs of anergy and exhaustion are initiated by the transcription factor NFAT, acting in the absence of its partner AP-1 (Fos-Jun). In the course of these studies, we developed an engineered NFAT, CA-RIT-NFAT1, which induces the characteristic features of exhaustion when transduced into CD8+ T cells. To understand the biological implications of this hyporesponsive program, we used mouse models of anti-tumor responses that involving adoptive transfer of tumor-reactive or unreactive TCR- transgenic T cells or T cells bearing chimeric antigen receptors (CAR-T cells). Using these models, we identified Nr4a transcription factors, and other families of transcription factors, as ?exhaustion-related? target of NFAT. We further showed that tumor-infiltrating CD8+ T cells lacking all three Nr4a family members displayed a gene expression profile similar to that of activated T cells and rejected tumors more efficiently than control CD8+ T cells. In this application we will test the hypothesis that mouse and human CD4+ and CD8+ tumor-infiltrating T cells (TILs) are functionally silenced by a cell-intrinsic transcriptional program mediated, at least in part, by NFAT and Nr4a transcription factors, and other transcription factors induced by NFAT. In Aim 1, we will define mechanistically the cell-intrinsic roles of Nr4a transcription factors in CD8+ T cell exhaustion. We will ask how Nr4a deletion in TILs overcomes exhaustion, enhances the effector phenotype and promotes tumor regression. In Aim 2, we will examine the roles of other pertinent transcription factors in CD8+ T cell exhaustion, and define the kinetics with which the exhaustion program unfolds. In Aim 3, we will examine the roles of NFAT and Nr4a in primary human T cells bearing exhaustion markers or transduced with CA-RIT-NFAT1, and in TILs isolated from human tumors. Our proposed studies will contribute to a broad mechanistic understanding of the transcriptional mechanisms operating in tumor-infiltrating immune cells, and may spark improved immunotherapies for cancer patients.